Some oil and gas wells are completed in unconsolidated formations that contain loose fines and sand. When fluids are produced from these wells, the loose fines and sand can migrate with the produced fluids and can damage equipment, such electric submersible pumps (ESP) and other systems. For this reason, completions for these wells can require sand screens for sand control. For hydrocarbon wells, esp. horizontal wells, the completion has screen sections with a perforated inner tube and an overlying screen portion. The purpose of the screen is to block the flow of particulate matter into the interior of the production tubing.
Even with the sand screen, contaminants and particulate matter can still enter the production tubing. The particulate matter usually occurs naturally or is part of the drilling and production process. As the production fluids are recovered, the particulate matter can cause a number of problems because the material is usually abrasive and can reduce the life of any associated production equipment. By controlling and reducing the amount of particulate matter pumped to the surface, operators can reduce overall production costs.
Some the particulate matter may be too large to be produced and may still cause problems at the downhole sand screens. As the well fluids are produced, for example, the larger particulate matter becomes trapped in the filter element of the sand screen. Over the life of the well as more and more particulate matter is trapped in the filter elements, the filter elements become clogged and restrict flow of the well fluids to the surface.
A gravel pack operation is one way to reduce the inflow of particulate matter before it reaches the sand screen. In the gravel pack operation, gravel (e.g., sand) is packed in the borehole annulus around the sand screen. The gravel is a specially sized particulate material, such as graded sand or proppant. When packed around the sand screen in the borehole annulus, the packed gravel acts as a filter to keep any fines and sand of the formation from migrating with produced fluids to the sand screen. The packed gravel also provides the producing formation with a stabilizing force that can prevent the borehole annulus from collapsing.
Horizontal wells that require sand control are typically open hole completions. In the past, stand-alone sand screens have been used predominately in these horizontal open holes. However, operators have also been using gravel packing in these horizontal open holes to deal with sand control issues. For example, FIG. 1A shows a borehole 10, which is a horizontal open hole, having a prior art gravel pack assembly 20 extend from a packer 14 downhole from casing 12. In the typical gravel packing operation, a screen 25 and a packer 14 are run into the wellbore together. Once the screen 25 and packer 14 are properly located, the packer 14 is set so that it forms a seal between wellbore and the screen 25 and isolates the region above the packer 14 from the region below the packer 14. The screen 25 is also attached to the packer 14 so that it hangs down in the wellbore forming an annular region around the exterior portion of the screen 25. The bottom of the screen 25 is sealed so that any fluid that enters the screen 25 can only pass through the screening or filtering material. The upper end of the screen 25 is usually referred to as the heel and the lower end of the screen 25 is usually referred to as the toe of the well.
To control sand in produced fluid from the borehole 10, operators attempt to fill the annulus between the assembly 20 and the borehole 10 with gravel (e.g., graded sand) by pumping a slurry of transport fluid and gravel into the borehole 10 to pack the annulus around the screen assembly 20. For the horizontal open borehole 10, operators pack the annulus using an alpha-beta wave (or water packing) technique, which uses a low-viscosity transport fluid, such as completion brine, to carry the gravel.
Initially, a washpipe 40 and crossover tool 30 are put together on an inner work string 45 at the surface and then run into the borehole to sting into the packer 14, pass through the packer 14, and run into the screen 20. The run-in of the washpipe 40 continues until the crossover tool 30 lands on the packer 14. The crossover tool 30 is usually dimensioned so that the packer 14 forms a second seal around the crossover tool 30 so that virtually no fluid is allowed to pass from above or below the packer 14 without passing through the ports 32 and 34 on the crossover tool 30.
After positioning the washpipe 40 into the screen 25, operators pump the slurry of transport fluid and gravel down the inner work string 45. The slurry passes through an exit port 32 in the crossover tool 30 and into the annulus between the screen 25 and the borehole 10 downhole from the packer 14. As the slurry moves in the annulus, the transport fluid in the slurry then leaks off through the formation and/or through the screen 25. However, the screen 25 prevents the gravel in the slurry from flowing back into the screen 25. The fluid returns passing alone through the screen 25 can then return through the crossover port 34 and into the annulus above the packer 14.
As the fluid leaks off, the gravel drops out of the slurry and first packs along the low side of the borehole's annulus. Traveling from the heel of the well toward the toe along the outside of the screen, the gravel collects in stages 16a, 16b, etc., which progress from the heel to the toe in what is termed an alpha wave. Because the borehole 10 is horizontal, gravitational forces dominate the formation of this alpha wave, and the gravel settles along the low side at an equilibrium height along the screen 25.
All the while, the transport fluid that carries the gravel drains inside the screen. As the fluid drains, pumping the slurry down the wellbore becomes increasingly difficult. Once a certain portion of the screen is covered, the gravel will start building back from the toe towards the heel in a beta wave, to completely pack off the screen from approximately its furthest point of deposit towards the heel. For example, the gravel begins to collect in stages (not shown) of the beta wave and forms along the upper side of the screen 25 starting from the toe and progressing to the heel of the screen 25. Again, the transport fluid carrying the gravel can pass through the screen 25 and up the washpipe 40.
To complete the beta wave, the gravel pack operation must have enough fluid velocity to maintain turbulent flow and move the gravel along the topside of the annulus. As the gravel fills back towards the heel, however, the open area to flow decreases, and the pressure on the formation increases. A high pressure area develops at the heel due to increasing pump pressure. Yet, the heel may be particular sensitive to pressure due to the type of formation involved because hard rock formations do not require a gravel pack. Instead, the types of formations needing gravel packing are typically sandstone, which has a much lower fracture gradient and a much lower compressive strength than a carbonite or shale reservoir. Oftentimes, the operators apply pump pressure at or near the fracture gradient of the formation with the completion brine hydrostatic pressure alone. Thus, as pressure is increased during the gravel pack operation, the operators may exceed the fracture gradient and may fracture the formation unintentionally. In these instances, well control can become an issue in addition to any damaging effects caused by losing fluid to the formation.
After the annular area around the screen has been packed with gravel, operators reposition the crossover tool 30 to reverse out. To do this, the ports 32 used for depositing the sand slurry into the annulus are raised above the packer 14, and the operators pump gravel free fluid down the annular area around the exterior of the workstring 45 to reverse the fluid inside of the workstring 45 back to surface. This pumping removes any the excess sand or gravel, but leaves the gravel that was placed around the exterior of the screen 25 in place.
Although the alpha-beta technique can be economical due to the low-viscosity transport fluid and regular types of screens that can be used, some situations may require a viscous fluid packing technique that uses an alternate path. In this technique, shunts disposed on the screen divert pumped packing slurry along the outside of the screen. FIG. 1B shows an example assembly 20 having shunts 50 and 52 (only two of which are shown). Typically, the shunts 50/52 for transport and packing are attached eccentrically to the screen 25. The transport shunts 50 feed the packing shunts 52 with slurry, and the slurry exits from nozzles 54 on the packing shunts 52. By using the shunts 50/52 to transport and pack the slurry, the gravel packing operation can avoid areas of high leak off in the borehole 10 that would tend to form bridges and impair the gravel packing.
Prior art gravel pack assemblies 20 for both techniques of FIGS. 1A-1B have a number of challenges and difficulties. During a gravel pack operation in a horizontal well, for example, the crossover ports 32/34 may have to be re-configured several times. The slurry pumped can sometimes dehydrate within the assembly's crossover tool 30 and associated sliding sleeve (not shown). If severe, settled sand or dehydrated slurry can stick the service tools and can even junk the well. Additionally, the crossover tool 30 is subject to erosion during gravel pack operations, and the crossover tool 30 can stick in the packer 14, which can create extremely difficult fishing jobs.
To deal with gravel packing in some openhole wells, a Reverse-Port Uphill Openhole Gravel Pack system has been developed as described in SPE 122765, entitled “World's First Reverse-Port Uphill Openhole Gravel Pack with Swellable Packers” (Jensen et al. 1009). This system allows an uphill openhole to be gravel packed using a port disposed toward the toe of the hole.